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    <p>I'd advise using <code>Map&lt;Lecturer, List&lt;Course&gt;&gt;</code>, as someone replied in the previous question, which means, "an association (<code>Map</code>) between different lecturers (<code>Lecturer</code>) to a list of the courses they teach (<code>List&lt;Course&gt;</code>), instead, which you would instantiate as <code>new HashMap&lt;Lecturer, List&lt;Course&gt;&gt;</code>. Otherwise, storing a list for each, you'll be duplicating functionality.</p> <p>Once you declared the <code>Course</code>s <code>c1, c2, ..., cn</code> a <code>Lecturer</code> <code>l</code> teaches, you associate them in the <code>Map</code> <code>m</code> as <code>m.put(l, c)</code> where <code>c</code> is a <code>List</code> of the courses, declared as <code>new LinkedList&lt;Course&gt;()</code>, and added as <code>c.add(c1); c.add(c2); ... c.add(cn);</code>.</p> <p>If I explain myself clearly.</p> <p>You can read <a href="http://java.sun.com/docs/books/tutorial/collections/interfaces/map.html" rel="nofollow noreferrer">http://java.sun.com/docs/books/tutorial/collections/interfaces/map.html</a> and <a href="http://java.sun.com/docs/books/tutorial/collections/implementations/map.html" rel="nofollow noreferrer">http://java.sun.com/docs/books/tutorial/collections/implementations/map.html</a> for more help on using <code>Map</code>s.</p> <p>To obtain the reverse association, you can easily use the following code:</p> <pre><code>Collection&lt;Course&gt; courses = m.values(); Map&lt;Course, List&lt;Lecturer&gt;&gt; reverseAssociation = new HashMap&lt;Course, List&lt;Lecturer&gt;&gt;; for (Course course : courses) { List&lt;Lecturer&gt; lecturersTeachingCourse = new LinkedList&lt;Lecturer&gt;(); for (Lecturer lecturer : m.keySet()) { if (lecturer.teaches(course)) { lecturersTeachingCourse.add(lecturer); } } coursesTaught.put(course, lecturersTeachingCourse); } </code></pre> <p>Which, as long as <code>Lecturer.teaches(Course)</code> queries whether the lecturer teaches the passed course, sets <code>reverseAssociation</code> as the courses-lecturers association. (Naturally, you should encapsulate that code as a method in Lecturers).</p> <p>Good luck JavaNoob! We were all there once!</p>
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    1. POAdding an ArrayList into a class - what does it do?
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    1. USHumphrey Bogart
    1. USHumphrey Bogart
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    1. COSo shouldnt I have a list of lecturers? cos i also need to be able to view the list of lecturers teaching a course. thanks your for ur reply
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    2. COIn principle you should avoid such circularity. I'll update my answer...
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